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This invention relates in general to the apportionment of torque between the axles of an automotive vehicle and, more particularly, to a torque biasing unit and a control system for it.
While most four real wheel automotive vehicles operate with the engine torque delivered to only the front wheels or the rear wheels, some vehicles, particularly the more sophisticated ones, are equipped with all wheel drive systems. In the typical vehicle so equipped, the engine torque is delivered to all four wheels all the time, but usually one set of wheels constitutes the primary driving wheels in that they are connected directly to the vehicle transmission, while the other set, known as the secondary wheels, is connected indirectly through a mechanism which accommodates for variations in velocity between the primary and secondary wheels. For example, when a vehicle negotiates a turn, the primary wheels follow arcs having radii different from the radii of the arcs followed by the secondary wheels, and as a consequence the primary and secondary wheels rotate at different angular velocities.
Some of these mechanisms have the capacity to apportion the engine torque between the primary and secondary wheels, with the apportionment depending on driving conditions. For example, a vehicle will accelerate best with most of the torquexe2x80x94and tractive effortxe2x80x94delivered through the rear wheels. On the other hand, when a turn is negotiated, it is best to transfer some of the torque to the front wheels. Certainly, when the primary wheels lose traction, more torque should be transferred to the secondary wheels. However, the mechanisms for apportioning torque are cumbersome and difficult to control. The typical mechanism involves a clutch having friction surfaces along which slippage occurs. This slippage is just one of multiple parameters involved in the control of the clutch.
The present invention resides in a transfer unit for apportioning torque between the primary and secondary axles of an automotive vehicle and a control strategy for the transfer unit. The transfer unit contains a torque bias coupling for extracting the torque for the secondary axle, and that coupling includes a clutch and a planetary gear set organized such that the extracted torque takes two paths through itxe2x80x94one a clutch path in which slippage can occur and the other a mechanical path in which no slippage occurs. The slippage in the clutch of the clutch path dissipates power, which manifests itself as heat, and the controller monitors the power dissipated to insure that it does not exceed prescribed limits for the conditions under which the automotive vehicle operates. The invention also resides in the process embodied in the operation of the transfer unit and controller.